Machines have been developed for forming a tensioned loop of thermoplastic strap around an object. Such machines typically include means for forming the loop about the object, means for pulling the strap loop trailing portion to tension the strap loop about the object, means for securing the overlapping strap portions together by melting and resolidifying regions of the strap, and means for severing the strap trailing portion from the loop.
One such machine is sold under the designation "MODEL MS POWER STRAPPING MACHINE" in the U.S.A. by Signode Corporation, 3600 West Lake Ave., Glenview, Ill. 60025, U.S.A. In this machine the strap is first manually looped around the object and the loop is then automatically tensioned. The overlapping strap portions are joined together after severing the trailing portion of the strap from the loop. This is effected with a heated member which moves from a retracted position spaced away from the overlapping strap portions to an extended position between the overlapping strap portions. The heated member moves in a direction generally perpendicular to the strap length. Although this works well in the applications for which it has been designed, the depth of the joint weld is not constant along the transverse cross sections of the overlapping strap portions. Since the heated member travels transversely relative to the width of the overlapping strap portions, the portions of the strap along one edge are necessarily in contact with the heated member for a longer period of time than the portions of the strap portions along the other edge of the strap. Although a central portion of the completed weld typically has the desired depth, the thickness of the weld at one edge may be less than desired while the thickness of the weld at the other edge may be more than desired.
Methods and apparatus have been proposed for welding overlapping strap portions together with a heated member moved between the overlapping strap portions in a direction generally parallel to the length of the strap. This eliminates the variation in the weld depth profile across the width of the overlapping strap portions. U.S. Pat. Nos. 3,368,323 and 3,397,105 disclose hand tools for effecting such a weld with a heated member. However, the disclosed hand tools do not automatically feed the strap around the object to be bound with the strap. Rather, the strap must be manually formed into a loop around the object, and the strap must be manually inserted in the proper position among the mechanisms comprising each hand tool.
It would be desirable to provide an improved method and apparatus for welding overlapping strap portions together with a heated member in an automatic strapping machine of the type having strap loop feeding and tensioning mechanisms.
It would also be advantageous to provide a method and apparatus for effecting a welded joint of overlapping thermoplastic strap portions with a heated member in a manner that would permit the width of the strapping machine to be reduced. This would permit conveyor systems to be located closer to the path of the strap loop and would permit multiple machines to apply strap to an object in a more closely spaced array.
Further, it would be desirable to provide an improved method and apparatus of the type described that would also produce a welded joint that is of substantially uniform thickness in transverse cross section at any selected point along the length of the welded joint.